897. Increasing Order Search Tree

Example 1:
Input: [5,3,6,2,4,null,8,1,null,null,null,7,9]

       5
      / \
    3    6
   / \    \
  2   4    8
 /        / \ 
1        7   9

Output: [1,null,2,null,3,null,4,null,5,null,6,null,7,null,8,null,9]

 1
  \
   2
    \
     3
      \
       4
        \
         5
          \
           6
            \
             7
              \
               8
                \
                 9  

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    TreeNode result;
    TreeNode curr;
    
    public TreeNode increasingBST(TreeNode root) {
        makeFlatInOrder(root);
        
        return result;
    }
    
    public void makeFlatInOrder(TreeNode node){
        if(node == null){
            return;
        }
        
        makeFlatInOrder(node.left);
        
        if(curr == null){
            result = new TreeNode(node.val);
            curr = result;
        }else{
            curr.right = new TreeNode(node.val);
            curr = curr.right;
        }
        
        makeFlatInOrder(node.right);
    }
}

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public TreeNode increasingBST(TreeNode root) {
        List<TreeNode> list = new ArrayList<>();
        
        inOrder(root, list);
        
        TreeNode node = new TreeNode(-1);
        TreeNode curr = node;
        
        for(TreeNode item : list){
            curr.right = new TreeNode(item.val);
            curr = curr.right;
        }
        
        return node.right;
    }
    
    public void inOrder(TreeNode node, List<TreeNode> list){
        if(node == null){
            return;
        }
        
        inOrder(node.left, list);
        
        list.add(node);
        
        inOrder(node.right, list);
    }
}

[문제] https://leetcode.com/problems/increasing-order-search-tree/